Hurricane Katrina roars into Louisiana

Hurricane Katrina eased up just a little before hitting the Louisiana coast of the US on Monday morning. But the hurricane picked a soft target - New Orleans has long been considered the US city at most risk from hurricanes.

Katrina formed in the Atlantic and reached hurricane strength just a few hours before hitting the Florida coast on Thursday night between Hallandale and North Miami Beach. Heavy rains flooded the low-lying area, and the hurricane briefly weakened on Friday morning before regaining strength over the warm waters of the Gulf of Mexico.

There Katrina grew into a powerful giant - almost 500 miles wide - and turned north toward Louisiana. On Sunday, sustained winds peaked at 175 miles per hour (280 km/h), making it a Category 5 storm - the highest on the scale. Its central pressure dropped to 902 millibars, making it the fourth lowest pressure ever recorded in the Atlantic Basin.

Only three Category 5 hurricanes have been recorded hitting the US. One of them, Camille in 1969, made landfall only about 120 miles (200 km) east of New Orleans. But this time the Gulf Coast was lucky, and Katrina weakened after mid-afternoon on Sunday.

The hurricane made landfall in Louisiana at 0610 local time (0710 EDT) as a Category 4 storm with sustained winds of about 140 mph (224 km/h). It swept north across New Orleans to hit the southern coast of the state of Mississippi. Katrina is the third Category 4 hurricane to hit land in 2005, in what is predicted to be one of the busiest hurricane seasons on record.

Catalogue of concerns

Preliminary reports cite flooding and collapsed buildings, but a full assessment of damage is impossible with the storm raging. A long catalogue of concerns must be addressed once Katrina passes over.

At the top of the list are the people who rode out the storm. Most of the 1.3 million residents of the New Orleans metropolitan area are thought to have obeyed evacuation orders. But nearly 100,000 people remained behind in the city - most of whom lacked any transportation. Some 9000 were packed into the Superdome, the most solid of the city's 10 hurricane emergency shelters, which lost pieces of its roof.

Flooding by a massive storm surge and heavy rains pose the biggest threat to the low-lying area. Levees line the Mississippi River and Lake Pontchartrain north of the city, but behind these subsidence of delta sediments has left some parts of New Orleans up to 20 feet (6 metres) below sea level.

Early reports say the storm surge was not as large as expected - one model predicted 26 feet (8 m). Officials were concerned that it could top the levees and overwhelm the pumps that normally keep the city dry.

Widespread flooding would be bad enough, but officials also worry it could be mixed with toxic materials from local oil refineries, urban sewage, and other detritus - even including bodies from local cemeteries.

Vulnerable marsh

Depending on the extent of flooding and damage to pumps and levees, it could be weeks or months before people could return to their homes. Several months could be required if decontamination was needed, and if affected structures were damaged beyond repair.

New Orleans is a petroleum industry hub, and many offshore wells operate in the region. Katrina halted work on many rigs and refineries, but the fear of extensive damage remains. Oil prices surged above $70 per barrel in early Monday trading.

The southern Louisiana coast is marshy and not heavily populated, but has been suffering heavy erosion for many years as underlying muds subside, and tens of square kilometres of marsh are replaced by open ocean annually.

Katrina's wind-driven waves are also likely to carve away large chunks of the vulnerable marsh, and may obliterate some coastal towns.

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